A single case of smallpox would trigger a federal emergency plan to vaccinate people close to the victim while detectives retraced the patient's every step over the previous three weeks.

Despite the likely panic over a disease not seen in the United States in half a century, the government would not resort to citywide mass vaccinations unless absolutely necessary.

The plan, obtained by The Associated Press, is a long-awaited set of step-by-step instructions for state health workers who would have to battle the highly contagious disease if bioterrorists ever unleashed smallpox.

The Centers for Disease Control and Prevention is putting final touches on the plan, work that was accelerated after the Sept. 11 terrorist attacks in New York, Washington and Pennsylvania. Officials say they consider the plan already operational, and have begun sending it to state health departments, who are supposed to begin the requested preparations now.

Some government officials are raising the possibility of one day resuming routine vaccinations of Americans against smallpox, which kills three in every 10 victims. Mass inoculation ended in the U.S. in 1972, almost a quarter-century after the disease last appeared in the United States.

The government has 15.4 million doses of smallpox vaccine and hopes to buy an additional 300 million. "It is the intention to determine, after we have sufficient supplies available to commence inoculation, to make the decision at that time," Homeland Security Director Tom Ridge said Thursday.

But Tommy Thompson, secretary of Health and Human Resources, said this week his department has no plans to implement a mandatory vaccination program and cited horrendous side effects as the principal reason.

Chilling photographs in the CDC document show children with severe, sometimes fatal, vaccine side effects, which illustrates why it would be a difficult decision to renew routine inoculations unless smallpox were to reappear.

"You're always hesitant to immunize people against the disease unless you're fairly certain that there is going to be a risk," Surgeon General David Satcher said.

The plan makes clear that the entire populations of cities or states will not be vaccinated unless CDC has evidence of more than a few cases. Instead, CDC will carefully apportion vaccine to family, friends, co-workers and others in close contact with a smallpox patient, as well as health workers and people who come in contact with a patient in a hospital.

It takes fairly close proximity, within about six feet of a person suffering the characteristic rash, to breathe in the smallpox virus and catch the disease. Quickly vaccinating those who live with or work around a patient is protective.

But, "a single case of smallpox would require an immediate and coordinated public health and medical response to contain the outbreak and prevent further infection," the plan's opening page warns.

Smallpox hasn't occurred in the United States since 1949; the world's last naturally occurring case was in Africa in 1977. When smallpox was declared eradicated in 1980, all research stocks of the virus were supposed to have been contained at the CDC's Atlanta laboratory and a similar lab in Russia. But the Soviet Union instead produced smallpox for their biological weapons program in the 1980s, and bioterrorism experts fear some may have spread to terrorist-sponsoring countries.

Smallpox symptoms include fever and a pock-like rash all over the body, which appears seven to 17 days after exposure to the virus. People are contagious from the time the rash appears, particularly in that first week of illness, until the scabs fall off.

The CDC's emergency plan would kick in when a doctor alerted state or federal health officials that a patient should be tested for smallpox, a test that can be confirmed only at CDC or at the Army's Fort Detrick, Md.

If smallpox were confirmed, he or she would be quarantined immediately. CDC Director Jeffrey Koplan would send vaccine from the government's stockpile while alerting the FBI and White House.

Disease detectives would grill the sick person and his family about every step taken over three weeks before becoming ill. They also would want addresses and phone numbers of every person who had close contact with the patient.

CDC then would have state health officials track down all of those people, vaccinate those who needed it, and do daily monitoring of whether they get a fever -- a sign they, too, might be sick with smallpox and require quarantine.

The plan instructs how to give the hard-to-handle vaccine. It is made with a live version of a virus related to smallpox, and thus can cause serious reactions in certain people, including people with faulty immune systems or those with the skin condition eczema. The document pictures a child covered in eczema-like lesions after contact with a recently vaccinated sibling. The worst reaction is a very rare -- one in 300,000 vaccinated babies -- but deadly encephalitis.